The present invention relates to a heat roll type fixing arrangement for an electrophotographic copier, laser printer, fascimile apparatus and other electrostatic recording apparatuses.
In an electrophotographic copier, for example, a toner image is produced on a photoconductive element by a predetermined process, then transferred to a paper, and then fixed on the paper. A predominant type of fixing device includes a fixing roll having a heater therein and a pressing roller pressed against the fixing roll to define a nipping section in cooperation with the fixing roll. A paper carrying a toner image thereon is moved through the nipping section so that the toner image is fused and thereby fixed on the paper. An inlet guide plate, or lower guide plate, and an upper guide plate are located to face each other for guiding the paper into the nipping section.
In a fixing device of the type described, while the paper is guided into thenipping section of the fixing and pressing rolls, it is apt to be creased in its opposite side portions and to curl upward in its trailing end portion. A toner image provided on a paper with such creases and/or curls is often disturbed. Some different approaches have been proposed to eliminate such an occurrence. One approach is using a generally hand-drum-like fixing roll whose diameter is slightly small at the center than at the opposite ends, so that a paper is prevented from being creased. Another approach is providing two ribs on a central part of a guide surface of an inlet guide plate such that the ribs individually extend in an intended direction of paper transport. When a paper is moved on and along the inlet guide plate, those ribs form folds in the paper in the transport direction to render the paper rigid and thereby prevents the trailing end portion of the paper from curling upward.
However, a problem with an inlet guide plate having the above configuration is that it fails to guide opposite side portions a paper of a relatively large size straightforward into the nipping section of the fixing and pressing rolls, i.e., the opposite side portions of such a paper abut against the periphery of the pressing roll and then enter the nipping section along the periphery of the pressing roll. In this condition, the side portions of the paper enter the nipping section slightly later than the central portion and are therefore creased. Further, when a post cart or like relatively thick and rigid paper is guided by the inlet guide plate toward the nipping section, the ribs deflect the leading end portion of the paper upward to prevent it from being accurately guided into the nipping section. This is apt to cause the paper to jam the transport path. It has been proposed to allow the inlet guide plate to selectively assume two different positions: a raised position adapted for an ordinary paper and in which the ribs protrude upward from the transport path, and a lowered position adapted for a post card or like relatively thick paper and in which the upper ends of the ribs are aligned with the paper transport path. This kind of scheme, however, adds to the cost of the fixing device. Although the distance between the front ends of the ribs and the inlet of the nipping section may be increased to admit even a relatively thick paper into the nipping section, the ribs raise the central portion of the paper above the opposite side portions and causes the former to abut against the fixing roll before the latter, again resulting in creases.